During drilling operations drilling fluid is pumped downhole to, among other things, provide hydrostatic pressure against the formation being drilled. The hydrostatic pressure provided by the drilling fluid in the annulus provides a static head which assists in maintaining the hydrostatic equilibrium in the wellbore, thereby controlling formation fluid pressure to prevent a significant influx of fluid from the formation (i.e., a kick) and minimizing fluid loss into and stabilizing the formation through which the well is being drilled.
In under balanced drilling, a rotating control device is used to close the borehole from external conditions, however the downhole pressure is maintained below a formation pressure. During underbalanced drilling fluid enter the formation due to the pressure gradient between the formation and the wellbore. While a small influx from the formation is tenable, a significant influx formation may slow drilling operations as well as increase the cost of drilling the well.
In managed pressure drilling, a rotating control device is used to close the borehole from external conditions, while a series of chokes and a back pressure system controls the downhole pressure in order to maintain a desired bottomhole pressure. A concern with managed pressure drilling and underbalanced drilling is the potential for a large fluid influx from a high pressure pocket in the formation, otherwise known as a “kick.” A kick is caused by the intrusion of salt water, formation fluids or gases into the drilling fluid which may lead to a blowout condition. Blowouts are hazardous, costly, and time consuming. A kick or blowout, if not mediated, may result in damage to drilling tools and systems and cause injury or death of workers.